Diodes provide a rectifying function which is necessary to the operation of many analog and digital circuits. There are many types of diodes which are classified primarily according to the special functions or characteristics thereof. For example, diodes can be fabricated to enhance such characteristics as power, speed, size, frequency response, forward and reverse breakdown voltages, etc.
A Schottky barrier diode is a specialized type of diode which is characterized by a low forward threshold voltage and exhibits a high switching speed. These characteristics are achieved in the formation of the diode by placing a Schottky barrier metal in intimate contact with a lightly doped semiconductor material. Because of the low work function at the metal-semiconductor interface, the diode is characterized by a lower forward breakdown voltage than otherwise obtained by a PN junction. This structure is also a majority carrier device, thereby substantially reducing minority carrier storage effects, and permitting high-speed switching thereof.
Schottky diodes can be employed as high speed rectifiers in many circuit applications, or can be used as clamping devices in conjunction with transistors to improve the switching speed thereof. When so utilized, a Schottky clamped transistor is formed. U.S. Pat. No. 3,909,837, by Kronlage, is exemplary of the fabrication of a Schottky diode formed across the base-collector junction of a bipolar transistor to increase the switching capabilities thereof.
In fabricating a Schottky diode according to conventional silicon semiconductor planar processes, certain artifacts can appear which degrade the quality of the diode performance. For example, when utilizing a Schottky barrier metal, such as platinum, as the anode material, and when utilizing aluminum as a terminal contact material overlying the Schottky anode material, a conductive diffusion barrier substance must be deposited therebetween to prevent the aluminum from penetrating or diffusing into the underlying platinum and thereby degrading the performance of the device. While the diffusion barrier material is effective to circumvent the noted problem, other anomalies may arise. As an example, during cleaning or deglazing of the Schottky anode metal surface, the insulation located peripheral to the anode metal area may also be removed, thereby exposing the underlying semiconductor material. Thus, when the conductive diffusion barrier material is deposited over the wafer and contacts the semiconductor material around the periphery of the Schottky anode metal, an additional lower anode metal-semiconductor secondary diode may be formed in parallel with the Schottky diode. A sub-optimal device is thereby formed, as the lower voltage diode short-circuits the higher voltage Schottky device.
From the foregoing, it can be seen that a need exists for a diode fabrication process which prevents the formation of the noted secondary diodes, and thereby provides improved performance and reliability of the Schottky diode. More particularly, a need exists for a Schottky diode process which prevents the undesired contact of diffusion barrier metal materials with the semiconductor material, and which encompasses no additional fabrication masks or masking steps.